CHEMISTRY COMMUNITY

In lecture today, Dr. Lavelle mentioned that, based on electron density, the shape of ammonia is trigonal pyramidal. But, this is not actually considered the molecule's shape. Can someone please explain this? Thanks!

Basically, we name molecule shapes based on the number of bonds they have, not based on the number of regions of electron density. So in terms of regions of electron density, ammonia is tetrahedral because there is one lone pair and three bonds (so four areas of electron density), but since we only consider the bonds when we name molecular shapes, and there are only three bonds, the molecular shape of ammonia is trigonal pyramidal.

Basically the electron density arrangement is what would give you your hybridization orbitals. So if your electron density arrangement is tetrahedral, then you would have sp3 hybridization. However, your molecular shape is actually based on the number of atoms bonded to the central atom, so if you have tetrahedral electron density arrangement, but one of those is a lone pair on the atom, so ammonia would be trigonal pyramidal in shape.